1. Field of the Invention
The present invention relates to an information reproducing apparatus for reproducing digital record information recorded on a CD (Compact Disc), a DVD (which is an optical disc having a record capacity equal to approximately seven times that of the CD) or the like.
2. Description of the Related Art
The CD or DVD is popularized which is an optical disc on which an audio signal, a video signal or the like is recorded as a digital record information. Moreover, a so-called CD-ROM (CD-Read Only Memory) or a DVD-ROM (DVD-Read Only Memory) is also popularized which is the CD or DVD as an external memory of a computer.
A reproducing apparatus for these optical discs receives a reflection light of a light beam irradiated onto the optical disc to obtain an analog reproduction signal corresponding to the digital record information, then corrects a high band attenuation property of the analog reproduction signal by an equalizer, then inputs it to an analog high pass filter to remove a direct current component contained in the analog reproduction signal, then converts it into a binary value and finally reproduces the digital record information.
Now, the inventor knows such a DVD-ROM reproducing apparatus as is schematically shown in FIG. 6.
As shown in FIG. 6, an information reproducing apparatus J is provided with a spindle motor 100, a spindle servo circuit 101, an optical pickup 102, an amplifier 103, an analog equalizer 104, an analog high pass filter 105, a comparator 106, a sampler 107, a PLL (Phase Locked Loop) 108, a decoding circuit 109, an error correction circuit 110 and an interface 111.
The analog high pass filter 105 is composed of a capacitor 112 and a resistor 113.
Next, a schematic operation will be explained below.
The spindle motor 100 rotates a DVD I at a predetermined rotation number under the control of the spindle servo circuit 101.
Then, the optical pickup 102 irradiates a light beam B onto the rotating DVD 1, converts the reflection light thereof into an electric signal so as to generates a reproduction signal Spp corresponding to the digital record information recorded on the DVD 1, and then outputs it to the amplifier 103.
Next, the amplifier 103 amplifies the inputted reproduction signal Spp at a predetermined amplification factor, and then outputs it to the analog equalizer 104.
The analog equalizer 104 emphasizes a high frequency band portion of the reproduction signal Spp, which originally has a high frequency band attenuation property, so as to correct a frequency property thereof.
The reproduction signal Spp in which the frequency property is corrected contains a low frequency noise component resulting from a variation in the reflectance and a variation in the refraction factor of the DVD 1, a trace error of a servo mechanism for the light beam B and the like. Therefore, the reproduction signal Spp in which the frequency performance is corrected is inputted to the analog high pass filter 105. The low frequency noise component containing a direct current component is removed by the analog high pass filter 105, so that the reproduction signal Spp is outputted as an analog reproduction signal Sp. In this analog reproduction signal Sp, a central level thereof coincides with a predetermined reference voltage.
Next, the comparator 106 compares a voltage of the analog reproduction signal Sp with the predetermined reference voltage (for example, a zero potential level), and then outputs a pulse signal (a pulse signal corresponding to the record information recorded on the DVD 1) So' of "1" or "0".
Accordingly, the PLL 108 generates a clock signal in which the phase is synchronous with the pulse signal So'.
The sampler 107 samples the pulse signal So' from the comparator 106 on the basis of the clock signal from the PLL 108, and then outputs digital data corresponding to the digital record information.
After that, the digital data is decoded by the decoding circuit 109 in accordance with a predetermined decoding method (for example, an 8/16 decoding method when the record information on the DVD 1 is reproduced). The error correction is performed by the error correction circuit 110. After that, the digital data is outputted through the interface 111 to an external equipment, for example, a host computer and the like.
Incidentally, assuming that a capacitance of the capacitor 112 is C and that a resistance of the resistor 113 is R, a cut-off frequency fc of the analog high pass filter 105 is expressed as following. EQU fc=1/2.pi.CR
Incidentally, in the field of the CD-ROM or the DVD-ROM, a multiple speed reproduction (e.g., a double speed reproduction, a triple speed reproduction and soon) for rotating it at a rotation speed of integer times of that for a normal music reproduction and reading out the recorded digital record information, and a variable speed reproduction for changing a reproduction speed depending on a position on a disc of the CD-ROM or the DVD-ROM are performed.
Now, the case of the DVD is considered as an example. If the digital record information recorded on the DVD is reproduced, the frequency band thereof (i.e., a frequency band of the reproduction signal Spp) is generally distributed in a frequency band from several hundreds of hertzes (Hz) to several mega-hertzes (MHz). In contrast with this, a frequency band of a noise unnecessary for the reproduction, such as external disturbance and the like, is distributed in a frequency band from a direct current to approximately tens of kilo-hertzes (kHz).
Thus, the noise resulting from the external disturbance and the digital record information to be originally reproduced exist together in the frequency band from hundreds of hertzes to tens of kilo-hertzes. When the conventional analog high pass filter is used, if the cut-off frequency is set high so as to perfectly remove the noise, the low frequency component of the digital record information is also removed. On the other hand, if the cut-off frequency is set low so as to perfectly pass the digital record information, the noise is also passed. After all, there is a problem that it is difficult to perform both of the removal of the noise and the pass of the digital record information.
Moreover, the digital record information is recorded on the DVD by a combination of pits having a plurality of lengths. At this time, for example, as shown in FIG. 7, if in a record sign Si as the digital record information, high levels each having a long length Pl are consecutive and then high levels each having a short length Ps are consecutive, the waveform of the record sign Si is as shown at a top stage of FIG. 7. Now, the dashed line at the top stage of FIG. 7 indicates the average of the values of the record sign Si. The average in the former half (i.e., a period in which the high levels each having the long length Pl are consecutive) is relatively high, and the average in the latter half (i.e., a period in which the high levels each having the short length Ps are consecutive) is relatively low. This variation of the average corresponds to the low frequency component of the digital record information. Incidentally, after the long pits are consecutive, the short pits are consecutive on the record surface of the DVD in this case.
Then, the reproduction signal Spp obtained in response to the record sign Si has the wave shown at a second stage of FIG. 7 by detecting the pits on the record surface of the DVD. Now, the dashed line at the second stage FIG. 7 indicate the average of the values of the reproduction signal Spp. In response to the above explained record sign Si, the average in the former half is relatively high, and the average in the latter half is relatively low. Moreover, black points on this average curve indicate the zero-cross points at which the reproduction signal Spp and a zero potential level intersect each other.
Then, the analog reproduction signal Sp generated by passing the reproduction signal Spp through the analog high pass filter 105 has the waveform shown at a third stage from the top of FIG. 7. Now, the dashed line indicates an average of the values of the analog reproduction signal Sp, similarly to the case of the reproduction signal Spp. Black points on this average curve indicate points corresponding to the zero-cross points of the reproduction signal Spp in the analog reproduction signal Sp.
As can be seen from these waveforms, since the analog high pass filter 105 removes the low frequency component from the reproduction signal Spp, the average of the analog reproduction signal Sp has the same constant zero potential level in both of the former half and the latter half. The zero-cross points (the black points) in the reproduction signal Spp corresponds to the negative level lower than the zero potential level of the analog reproduction signal Sp in the former half, and corresponds to the positive level higher than the zero potential level of the analog reproduction signal Sp in the latter half. That is, the location situated at the zero-cross point in the stage of the reproduction signal Spp is shifted to the positive side or the negative side in the analog reproduction signal Sp.
Thus, a pulse signal So' having a waveform shown at a bottom stage of FIG. 7 is generated by detecting by the comparator 106 whether this analog reproduction signal Sp is higher or lower than the zero potential level, that is, just positive or negative and then reproducing it. However, a length Pl' or Ps' of a high level period in this pulse signal So' has a length different from that of the length Pl or Ps of the high level period in the record sign Si as indicated by FIG. 7.
In other words, this implies a problem that information different from the digital record information to be reproduced is in fact reproduced because the analog high pass filter removes the low frequency component from the digital record information.
Moreover, while the frequency band of the analog reproduction signal to be reproduced is also changed in the above mentioned multiple speed reproduction or variable speed reproduction, the cut-off frequency is constant in the conventional analog high pass filter. Hence, this results in a problem that the noise component cannot be effectively removed in response to the variation of the frequency band.